Low-Cost/Power Coherent Transceivers for Intra-Datacenter Interconnections and 5G Fronthaul Links

Currently there are multiple approaches considered by the optical communications community in order to scale-up the transceiver bit-rates beyond the Tb/s mark. Those include implementations based on higher baud-rates, advanced modulation formats and some form of parallelism, either by using Parallel Fibers Multiplexing (PSM) or Wavelength Division Multiplexing (WDM), or both. However, the cost and power consumption of the resulting transceivers, normalized per Gb/s, do not drop linearly as the number of parallel lanes (utilizing either spatial or spectral resources) increase (due to the extra complexities associated with multiplexing/demultiplexing and wavelength control). The cost and power consumption targets for the associated transceivers are particularly stringent for the case of intra-datacenters connectivity and 5G fronthaul links, unlike the cases of inter-datacenter connectivity and terrestrial/submarine networks. For intra-datacenter and 5G fronthaul interconnections, in order to reduce the cost and power per bit, it would be ideal to develop solutions operating on a single I/O fibre and based on single-wavelength implementations, thus meeting the reach and bit-rate requirements using a single transceiver at each end.Coherent detection could in principle offer a nice solution in that direction, enabling to scale the transceivers towards higher capacities; however, the cost and power consumption issues stemming mainly from the use of ultra-high-speed DSP chips and analogue to digital converters (ADCs), which consume over 8 W, even at 7 nm technology, must be addressed. Thus, Coherent transceivers that could circumvent the use of DSP chips, as well as eliminate DAC/ADCs, have high future potential, even though, at the moment, they seem inferior in terms of transmission performance to the existing solutions (utilizing high-speed electronics DACs/ADCs and DCP chips). However, the performance requirements for intra-datacenter and 5G fronthaul transceivers are not very stringent, as compared to the case of longer-reach transceivers. This implies that some of the simpler transceiver designs (bypassing costly and power-hungry high-speed digital electronic circuits) could find their way into commercialization for intra-DCI and 5G fronthaul networks. In our presentation, we survey the different options proposed in the literature for DSP-chip-less coherent transceivers and then we focus on the implementation principles and benefits of a new implementation entirely based on all-optical signal processing.